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    <title>Document</title>
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    <script>
        // 找出数组中 元素为10的下标，有则打印该下标，没有则打印 - 1
        //       - 例如: [88, 20, 10, 100, 50]  打印 2
        //       - 例如: [88, 20, 30, 100, 50]  打印 - 1
        let arr = [88, 20, 10, 100, 50]
        let a = 0
        // 遍历数组
        for (let i = 0; i < arr.length; i++) {
            if (arr[i] == 10) {
                a = i
                break
            } else {
                a = -1
            }
        }
        console.log(a);
    </script>
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